After having rented this same video over and over again at least a dozen times from our local video store, I decided that the time had come to just buy the thing, which I should have done from the second time my son picked it out from the shelf (and saved all the rental fees).
I think this is one of those "feel good" videos for kids - at least it is for my son. It may not be educational (although I think there is some educational benefit in teaching kids about different personality traits, different moods, and the finer points of social interaction), but it does appeal to young kids' need to have the world be a place that makes sense.
In Dillydale (the town where Mr. Men characters all live), everything is spelled out - the character who is always a grouch is named Mr. Grumpy, the one who is clumsy is named Mr. Clumsy, etc. Kids can safely rely on the different characters behaving the same way all the time - which helps them feel accepting of them. So even though Mr. Fussy is incredibly annoying, he is endearing because he is acting the way the audience expects him to act. This leads to kids learning to accept behaviors in themselves and others in ways that are not judgemental or condemning, which is probably a good thing. It also creates a world where everything makes sense, which is good for little ones to encounter, especially when so much of the world around them is so incredibly complicated and makes no sense at all for their young minds to comprehend.
Of course this is all just my own guesswork, and I'm not in any way a professional to weigh in on this, but it's my way of trying to explain to myself why this series is so incredibly appealing to kids, including my own son.